In some environments where undesired substances are removed from a gas, such as air, by filtering the gas, it is important to continuously check that the filter is working and detect any leakage of the filter. One way of checking the filter is to use a test probe, by means of which gas is collected downstream of the filter and analyzed with respect to the undesired substance. Since it is desirable that the test probe does not noticeable interfere with the gas flow it cannot cover the whole area. Different concepts of test probes which are moved to scan the filter area have been developed. One kind thereof is an elongated test probe, which extends along the length or width of the filter and is moved back and forth perpendicularly of its longitudinal extension to scan the area in the vicinity of the filter surface. A typical elongated test probe is made of a tube with several inlet holes through the tube wall distributed along the length of the test probe, and a central outlet. However, this kind of test probe is subjected to problems with a nonlinear intake of air through the inlet holes, wherein the gas flow velocity through an inlet hole depends on the distance from inlet hole to the outlet, and thus differs along the length of the test probe. This in turn causes a lower sensitivity to leakage at the edges of the filter compared to a more centrally positioned leakage.
U.S. Pat. No. 7,658,787 discloses a test probe having several inlet openings, called sample ports, each being funnel shaped towards a respective outlet tube. The sample ports are arranged side by side along the probe. This probe design partly solves the problem, but at the prize of a large number of tubes. An alternative solution would therefore be desirable.